The present invention relates to a catalyst for purifying an exhaust gas, catalyst which is used in an exhaust system of an automobile to purify harmful components in the exhaust gas, more particularly to an HC-adsorption type NOx-selective-reduction type catalyst that can selectively reduce and remove NOx in an oxygen-rich exhaust gas, which is emitted from a diesel engine, or the like, by HC adsorbed on a zeolite support.
As an NOx-selective-reduction type catalyst that selectively reduces NOx and purifies them, a catalyst for purifying a diesel exhaust gas has been known which is made by loading a catalyst metal on a zeolite. Since the zeolite has many acidic sites so that it is acidic, it is good in terms of the HC adsorbing capability and adsorbs HC in the exhaust gas. Therefore, even in an oxygen-rich exhaust gas, such as the diesel exhaust gas, the atmosphere becomes a stoichiometric or a rich atmosphere, in which HC are abundant, adjacent to the catalyst, and NOx react with the adsorbed HC and are thereby reduced and purified by the catalytic action of the loaded catalyst metal.
Further, the zeolite exhibits a cracking action, and zeolites, such as mordenite, ZSM-5, an ultra-stable and type xe2x80x9cYxe2x80x9d zeolite (US-Y), exhibit an especially high cracking action. Therefore, by using these zeolites as a catalyst support, the SOF (Soluble Organic Fraction) in the diesel exhaust gas are subjected to cracking, and become low-molecular HC, which are more likely to react, and thus the NOx can be reduced and purified more efficiently.
Furthermore, by injecting HC, such as a light oil and propane, into the diesel exhaust gas, the exhaust gas atmosphere is turned into a stoichiometric or a rich atmosphere, and thereby the NOx purifying capability is enhanced. In this case as well, by using a zeolite support, the atmosphere adjacent to the catalyst becomes a rich atmosphere in which HC are more abundant, and accordingly the NOx purifying capability is further improved.
In HC-adsorption type catalysts in which a zeolite is used as a support, improving the HC adsorbing capability would result in improving the NOx purifying capability. However, according to recent studies, it has been revealed that upgrading the HC adsorption capability does not necessarily result in improving the NOx purifying capability.
One of the causes for this is that the so-called HC poisoning of the catalyst metal occurs when the adsorbed HC are desorbed to adsorb onto the catalyst metal, such as Pt, or the like, and the activity of the catalyst metal decreases.
Further, the zeolite is a tectaluminosilicate chemically, and zeolites having a variety of Si/Al ratios have been known. And it has been understood that the characteristics of zeolites depend greatly on the values of this Si/Al ratio.
Since a zeolite having a small Si/Al ratio has many acidic sites and exhibits a high cracking capability and a high HC adsorption capability, a catalyst made by loading a catalyst metal on this is good in terms of the NOx purifying capability. However, in the zeolite having many acidic sites, the HC adsorbed in the pores are carbonized to cause the caulking easily, and they close the insides of the pores, and as a result, there is a drawback in that the HC adsorbing capability decreases chronologically.
Further, a zeolite having many acidic sites loses the acidic sites easily by de-Al (The 4-coordination is turned into the 6-coordination in the zeolite structure.) when it is subjected to a hydrothermal durability test, there arises a drawback in that the cracking capability decreases. Furthermore, in the catalyst in which a catalyst metal is loaded on such a zeolite, by the de-Al resulting from the hydrothermal durability test, the catalyst metal grows granularly so that the activity decreases considerably and there arises a drawback in that the durability is low.
Whilst, a zeolite having a large Si/Al ratio has advantages in that, since it is less likely to be subjected to the de-Al and since the granular growth of the catalyst metal resulting from the de-Al is suppressed, it is good in terms of the durability. However, in the zeolite having a large Si/Al ratio, since the acidic sites exist less, the adsorbing capability and the cracking capability are low so that there is a problem in that the NOx purifying capability is low initially.
By the way, sulfur oxides (SOx), which result from the combustion of the sulfur components in the fuel, are contained in the exhaust gas, and there arises a drawback in that the catalytic activity of the catalyst metal decreases when these SOx components adsorb onto the catalyst metal (especially, Pt). This is referred to as the SOx poisoning.
As a method for inhibiting the SOx poisoning, it is possible to think of a method in which an alkaline component, such as an alkali metal and an alkaline-earth metal, is loaded together with the catalyst metal. Namely, since SOx is more likely to adsorb onto the alkaline component than onto the catalyst metal, such as Pt, the SOx poisoning of the catalyst metal is suppressed, and the high catalytic activity is maintained.
However, when the alkaline component is loaded, the SOx poisoning of the catalyst metal is suppressed, but the alkaline component is loaded onto the acidic sites of a zeolite preferentially. Therefore, the neutralization results in the disappearance of the acidic sites, and there arises a drawback in that the major characteristics of a zeolite, i.e., the HC adsorbing capability and the cracking capability, decrease.
The present invention has been developed in view of these situations, and it is a major object of the present invention to make an HC-adsorption type exhaust-gas-purifying catalyst which exhibits a high NOx purifying capability initially and which can maintain the high conversion after a durability test.
Further, it is a secondary object of the present invention to further improve the NOx purifying capability by suppressing the HC poisoning of the catalyst metal while improving the HC adsorbing capability.
Furthermore, it is a further object of the present invention to suppress the SOx poisoning of the catalyst metal, and also to suppress the decrement of the high HC adsorbing capability and the high cracking capability of a zeolite.
It is a characteristic of an exhaust-gas-purifying catalyst of embodiments of the invention that it is a catalyst reducing and purifying nitrogen oxides in an oxygen rich atmosphere which contains oxygen more than necessary for oxidizing components to be oxidized in the exhaust gas by hydrocarbon (HC) adsorbed on a zeolite support, wherein:
the zeolite support comprises a first zeolite loaded with at least a catalyst metal selected from the group consisting of Pt, Rh, Pd, Ir and Ag and at least an alkali component selected from the group consisting of alkali metals and alkaline-earth metals and a second zeolite free from loading a catalyst metal and an alkali component, and both the first zeolite and the second zeolite are mixed in a powdered state.
In further embodiments, in the exhaust-gas-purifying catalyst, the first zeolite has pores whose diameters are a predetermined value or less and the second zeolite has pores whose diameters exceed the predetermined value.
In further embodiments, in the exhaust-gas-purifying catalyst, the predetermined value is 5.5 xc3x85.
In further embodiments, in the exhaust-gas-purifying catalyst, the first zeolite is xc2xc-xc2xd by weight ratio in all zeolites.
In further embodiments, in the exhaust-gas-purifying catalyst, a molar ratio of silicon with respect to aluminum (Si/Al) is larger in the first zeolite than in the second zeolite.
In further embodiments, the exhaust-gas-purifying catalyst is reducing and purifying nitrogen oxides in an oxygen rich atmosphere which contains oxygen more than necessary for oxidizing components to be oxidized in the exhaust gas by hydrocarbon (HC) adsorbed on a zeolite support, wherein: the zeolite support comprises a first zeolite loaded with at least a catalyst metal selected from the group consisting of Pt, Rh, Pd, Ir and Ag and a second zeolite free from loading a catalyst metal, a molar ratio (Si/Al) of silicon with respect to aluminum of the first zeolite is greater than that of the second zeolite by 150 or more, and both the first zeolite and the second zeolite are mixed in a powdered state.
In further embodiments, in the exhaust-gas-purifying catalyst, the first zeolite is ⅔-xc2xd by weight ratio in all zeolites.
In further embodiments, in the exhaust-gas-purifying catalyst, the first zeolite employs mordenite exhibiting 200 or more, ZSM-5 exhibiting 1,000 or more, type xe2x80x9cYxe2x80x9d zeolite exhibiting 400 or more, type xe2x80x9cAxe2x80x9d zeolite exhibiting 400 or more, ferrierite exhibiting 400 or more, or zeolite xcex2 exhibiting 200 or more by a molar ratio (Si/Al).
In further embodiments, in the exhaust-gas-purifying catalyst, the first zeolite has pores whose diameters are a predetermined value or less, and the second zeolite has pores whose diameters exceed the predetermined value. And, in further embodiments, in the exhaust-gas-purifying catalyst, the predetermined value is 5.5 xc3x85.